It is often necessary to print large area electrical circuits with conductors having at least one lateral dimension of 1-1000 microns. One process for accomplishing this type of circuit printing is using vacuum deposition. This method, however, is a high-cost operation and is usually only suitable for batch processing.
Another method of constructing electrical circuits is inkjet printing of patterns using metal nanoparticles to form conductors. This process is discussed in S. Molesa et al.; “High-quality inkjet-printed multilevel interconnects and inductive components on plastic for ultra-low-cost RFID applications” University of California, Berkeley. Some problems associated with this technique are that it is substrate dependent, it is difficult to achieve lateral dimensions of less than 100 microns, and the particles must be annealed by bulk heating, which can cause substrate deformation. Another problem with inkjet deposition is that it often requires multiple passes to deposit the proper amount of material, which reduces throughput.
Attempts to solve the bulk-heating problem, shown in the following two references, involve using high-powered lasers to anneal nanoparticles. N. R. Bieri et al.; “Microstructuring by printing and laser curing of nanoparticle solutions” Applied Physics Letters, Volume 82, Number 20, May 19, 2003, pages 3529-3531; and J. Chung et al.; “Conductor microstructures by laser curing of printed gold nanoparticle ink” Applied Physics Letters, Volume 84, Number 5, Feb. 2, 2004, pages 801-803.
Lithographic patterning techniques have been employed in conventional fabrication of microelectronic devices, including thin film transistors (TFT) arrays for flat panel application. Conventional photoresist lithographic techniques applied to microfabrication have proved capable of defining structures and forming regions of material on a substrate to within dimensions of about 100 nm.
Based on a printing model, the lithographic process forms a pattern of areas that are either receptive or repellent (non-receptive) to a coating (such as ink) or to some other treatment. Conventional photolithography requires a small number of basic steps, with variations according to the materials used and other factors. A typical sequence is as follows:                (i) wet coating of a positive-working or negative-working photoresist (such as by spin-coating);        (ii) prebake of the photoresist;        (iii) exposure by some form of electromagnetic radiation through an overlay mask using an optical mask aligner to form the pattern;        (iv) curing of the masked pattern, such as by postbake; and        (v) removal of the uncured portion, using a liquid to form a pattern.        
Following subsequent coating or treatment of the surface, the protective photoresist pattern can then itself be removed.
Steps (i)-(v) may be performed in air, such as in a clean room environment, and are typically performed using separate pieces of equipment. Alternately, one or more steps, such as coating deposition, may be performed in vacuum. Because of the very different nature of processes carried out in each of these steps, it would not be readily feasible to combine steps (i)-(v) in any type of automated, continuous fabrication system or apparatus.
Considerable effort has been expended to improve upon conventional methods as listed in steps (i)-(v) above in order to achieve better dimensional resolution, lower cost, and eliminate the use of chemicals such as etchants which may interact with layers previously applied.
As is well known to those skilled in the microlithographic art, conventional photoresist materials follow a “reciprocity law,” responding to the total exposure received, the integral of illumination over time. Conventional photoresists are typically exposed with light in the UV portion of the spectrum, where photon energy is particularly high. Examples of photoresists used microfabrication of semiconductor components are given in U.S. Pat. No. 6,787,283 (Aoai et al.).
Additional advantages to conventional process described above are provided by e-beam and X-Ray lithography in that they provide a partial reduction of chemical processing, and while X-Ray lithography still requires the use of masks, e-beams can be used to write patterns in a resist directly without a mask. High energy radiation sufficient to cause bond breaking in organic materials causes chain scission, or depolymerization, in a coated resist such that it can be removed in the image area with solvents that will not remove the non-radiated areas. E-beams, when used as a direct pattern writing device, suffer from low throughput due to long scan times as a result of the serial limitation of a single beam exposure, and are thus limited to low volume manufacture. E-beams are used primarily in the microfabrication industry to manufacture masks for conventional processes due to their sub-micron high resolution capability.
Another improvement for metal patterning is the lift-off process, which is well known in the art. This process avoids the need for an etchant but at the cost of complexity. In this process a patterned resist layer is formed with an overhang. The patterned material is then subject to evaporation or sputtering of the desired material to be patterned. The overhangs will shadow edge regions of the resist assuming the evaporation or sputtering process is approximately collimated. This shadowed region acts as breaks in the deposited material.
Application of a resist solvent removes the resist allowing the overlying deposited material to be removed. The breaks allow the material to separate, and allows the solvent to enter and dissolve the resist.
There is a need for a process which avoids the complexity of forming overhangs, does not require a collimated deposition process and does not require a vacuum system.
A further improvement is provided by direct ablation of a resist with a high energy laser at wavelengths less than 400 nm with energies sufficient to cause resist bond breaking, volatilization, and material evacuation of the resist in the irradiated areas, thus making the solvent development step unnecessary. However, the laser systems for direct UV ablation are quite expensive, pulse, difficult to maintain, and suffer from low throughput due to their single beam limitations. Large area excimer lasers solve that deficiency, but they suffer from the requirement of a mask to form the pattern.
Therefore, a need exists for a method of direct writing, or maskless lithography that allows for the use a less expensive and versatile class of laser directed radiation, specifically the solid state IR diode lasers. IR diode lasers offer the advantages of cost, availability, reliability, and lifetime, and are used widely in the communications industry, in a variety of electronic devices such as CD and DVD writers and players, and in the graphic and reprographic arts including digital color printers, laser thermal platewriters, imagesetters, and proofers. In addition, the individual lasers can be joined in an array of up to almost one hundred or more separately modulated lasers dramatically increasing throughput compared to single beam devices. Alternatively, the light can be conjoined from several laser sources into a single bar laser fitted with a segmented light gate modulator of between 200 to up to 1000 separate addressable channels of individually controlled beams. The beam dimensions are limited only by the wavelength of the light they deliver, and can produce spots as small as 2 microns in the array direction as defined by the spatial light modulator. Examples of commercial laser systems with such capability are the Kodak Newsetter and the Creo Trendsetter plate-writer-proofer. Feature resolution of 2 microns is therefore possible with such diode laser array systems, which is more than sufficient for thin film transistor array backplanes and color filter arrays used in LCD and OLED displays. These IR lasers, as well as YAG lasers that operate in the visible spectrum, suffer from photon energies less than sufficient to break organic bonds and effect direct ablation of resists.
A need exists for a maskless lithographic method for microstructure construction that limits the need for wet chemical etching, in order to reduce cost and to be compatible flexible support substrates and roll to roll continuous manufacture.